Such linear motors usually have ball bearings or plain bearings for radial support and for dissipating the radial forces. The requirement for these linear stepper motors is to be able to position the actuating rod as precisely as possible both radially and axially, i.e. specifically the axial play of the spindle relative to the spindle nut and the axial play of the whole spindle in the housing should be as small as possible. As a rule, total axial plays ≥0.3 mm are pursued. This requires very small tolerances of the components in manufacture. Added to this, under cool temperatures, the stator contracts much more than the spindle mounted in its interior. Therefore, a maximal axial play must be maintained so that the spindle does not jam in cool temperatures, otherwise the actuating force of the linear stepper motor can decline significantly.
A generic linear stepper motor is known from DE 103 32 389 A1, where the outer web ends of the actuating rod of this linear drive are of such a length that they can be fitted through corresponding guide cut-outs of the D-side bearing shield. After fitting through, a coupling shank that closes the web ends of the actuating rod is fitted and suitably fixed. The housing is bounded by two bearing shields, where two plain bearings for radial support and two balls for axial support of the spindle are arranged in both bearing shields by means of plain bearing journals of appropriate length. A problem here is the wobbling movement of the bell-shaped rotor induced by the two plain bearings and the precise adjustment of axial play.
According to EP 1 414 636 B1 an electro-mechanical linear drive with electric motor, having a hollow shaft rotor open on one side and a helical gear located in the interior of the hollow shaft rotor and converting the rotational movement of the hollow shaft rotor into a linear movement, operates a spindle shaft, wherein the rod-shaped end of a sleeve-shaped spindle nut protrudes from the open D-side of the linear drive.
EP 1 928 074 B1 describes a linear actuator comprising a common motor design in a separate two-piece housing with internal rotor with rotor shaft, a stator with coil, the rotor being radially and axially supported by a bearing on the motor side. Directly following the rotor shaft a threaded shank is formed on which a screw-nut system is arranged for producing a linear movement. This linear displacement unit is designed as a fork shape and consists of a nut portion and a coupling portion for its connection with a unit to be controlled. Furthermore, the motor shaft is axially supported by two balls, the balls bearing against separately formed hard stops. Moreover, this design additionally has a spring arranged in a bearing intended to prevent axial play of the rotor shaft with the following spindle. This design is characterised by a large number of components and has a relatively long structure.
Another linear drive with a claw-pole stepper motor is shown in DE 10 2008 054 330 A1, where the motor housing is also a radially split two-piece design. After assembly of the motor components, both motor housing halves can be locked and positioned against each other by means of bayonet joint. In this drive solution, too, the linear motor consists of an elevated number of components. To provide radial support, two radial grooved ball bearings are internally mounted.
DE 10 2005 055 868 B4 describes another threaded drive for a linear servo-motor with a flanged two-piece housing, designed as a very short structure, since the threaded spindle is supported and guided by two ball bearings inside the linear servo-motor. However, this motor also consists of a relatively large number of components, which makes its manufacture unnecessarily costly.